Photoelectric device



April 13, 1937. J. H. DE BOER ET AL 2,076,622

I PHOTOELECTRIC DEVICE Filed May 11, 1951 INVENTORS JAN HENDMK De BOERgum CHEISTIAQN VAN GEEL ATTbRNEY Patented Apr. 13, 193? PHOTOELECTRICDEVIQ'JE Jan Hendrik de a l and Willem Cstiaan van Geel, Eindhoven,Netherlands, assignors to Radio Corporation of America, a corporation ofDelaware Application May 11, 1931, Serial No. 536,647 In the NetherlandsMay Iii, 19,30

14 Claims.

(Granted under the provisions or see. It, act-of March 2, 1927; 357 O.G. 5)

The invention relates to photoelectric devices, 1. e., devices with theaid of which certain-electrical phenomena can be obtained by means ofrays of light.

A photoelectric device according to the invention comprises an electrodeconsisting, at least partly, of a photoelectric substance, and separatedby a layer comprising one or more insulating solid substances from asecond electrode consisting of electrically conducting material. By aphotoelectric substance is meant hereinafter a substance which whenirradiated by light rays emits electrons. I

When the photoelectric electrode is irradiated, it appears that apotential difference is produced between the two electrodes of thedevice. If these electrodes are connected to one another via agalvanometer, this tension causes a current to fiow through the saidmeter. The value of the tension set up and, therefore, of the currentflowing through the galvanometer has been found to be dependent on theintensity of the light with which the photoelectric electrode isirradiated.

The photoelectric electrode is preferably made of one or more alkali-oralkaline-earth metals although good results can also be obtained forexample with cadmium which, irradiated with ultra-violet rays, isphotoelectric.

The separating layer can be formed between the two electrodes indiiierent ways. In many cases the manufacture of the device can besimplified by causing the separating layer to consist of a chemicalcompound of the metal from which one of the electrodes is formed.

The sensitiveness of the device can be increased bycausing thephotoelectric substance, at least partly, to adsorb to the separatinglayer. In this case it is not absolutely necessary that the wholeseparating layer should consist of a substance which is a good adsorbentof the photoelectric substance. It isalso possible to cause this layerto consist of such substance only at the surface which comes intocontact with the photoelectric electrode.

.In many cases the value of the potential difference set up between theelectrodes due to the' irradiation may be favourably influenced bymaking the photoelectric electrode so thin that it is transparent tolight, so that the rays can penetrate as far as the boundary surfacebetween the photoelectric electrode and the insulating substance. Tothis end the photoelectric electrode may conveniently consist of amono-molecular layer of a photoelectric substance.

device according to the invention. ing, Figures 1 and 2 represent twodiiierent ele- Sometimes it is desired to decreasethe electricresistance of the separating layer. This can be advantageously eifectedby introducing into the separating layer particles of an electricallyconducting substance. Care should of course, be taken not to introduceso many conducting particles into the layer that the electrodes of thedevice are short-circuited. The conducting particles may advantageouslyconsist of particles of the photoelectric substance.

The invention will be more clearly understood by'referring to theaccompanying drawing which represents, by way of example, aphotoelectricIn this drawvations of the said device.

The device represented has a sealed vitreous envelope l consisting ofglass, quartz or similar transparent material and having sealed to it'astem 2 on which are mounted the electrodes of the device. One of theseelectrodes is a'metal plate 3, which may, for example, be zirconium,secured by means of a supporting wire 4 to the stem of the tube, saidsupporting wire being connected to a leading-in wire 5.

The electrode 3 is provided with a thin layer 6 of particles ofinsulating material. This layer 6 can be produced in a simple manner byoxidizing the zirconium plate 3, which may be effected prior to mountingthe plate 3 inside the envelope I. Owing to this oxidation the plate 3is coated at its surface with a layer of particles of: zirconiumoxide,.which, as is commonly known, is electrically insulating. Thislayer is preferably of a thickness of about 1 micron and at any rateconsiderably less than 1 mm.

After the electrode 3 with its layer 6 has been mounted inside theenvelope, the latter is exhausted, which may bedone with a vacuum pumpconnected to the stem tube 1. After the exhaustion some caesium isintroduced into the envelope. The caesium vapor may be developed outsidethe envelope, for example, in a chamber connected to the supply wire ll.After the formation of the photoelectric electrode, the envelope I mayif desired be filled with an inert gas or vapor. j

When the formed caesium fllm 9 is irradiated, for example, with the aidof a source of light l2,

a potential difference is set up between the electrodes 3 and 9, whichmay cause a current to flow through a device connected between the wires5 and H For examplea galvonometer thus connected showed currents of somemicro-amperes when the device was irradiated with a beam of light havingan intensity of about 0.5 lumen/ square cms. The potential differenceset up is dependent on the intensity of the light, so that the devicecan be utilized for converting light variations into electric voltage orcurrent variations. In contradistinction tothe photoelectric cells knownup to the present it is not absolutely necessary for the use of a deviceaccording to the invention to connect a separate source of voltagebetween the electrodes. r

The polarity of the potential difference set up and, therefore, thedirection of the current pro duced by this potential difference depends,inter alia, on the point of lighting and on the thickness of thezirconium oxide layer 6. This may be made clearer by describing somephenomena perceived with the above-described device. In one experimentthe caesium electrode 9 was irradiated by a source of light whose rayscould be concentrated on a small portion of the electrode 9. The beam oflight striking this electrode had a cross-section of several squaremillimeters and the :electrode was three centimetres in width and alsoin length. Moreover, the resilient lead wire l0 was bearing at the edgeof the electrode system on a portion of' the zirconium oxide layer whichhad -a greater thickness than the other portion of this layer. Thisthicker portion of the oxide layer had a white colour and is denoted inFigure 2 by B whereas the thinner portion of the oxide layer, which hada green colour, is denoted by A. The whole film of zirconium oxide wascovered by the caesium film 9.

If the beam of light struck the caesium layer covering the portions A ofthe zirconium oxide layer, there was produced between the electrodes 3and 9 such a potential diflerence that after the wires 5 and ii had beenconnected to a galvanometer', an electron current flowed from thephotoelectric electrode 9 through the lead wire iii, the galvanometer,the support wire 4 to the electrode 3. For the external circuit thephotoelectric electrode 9 and the electrode 9 consequently constitutedthe negative and the positive poles respectively.

If the beam of light-struck the photoelectric substance covering theportion B of the zirconium oxide it was observed that with the samelight intensity a heavier current flowed in the opposite direction..This current appeared to have a maximum intensity when irradiating witha wave length of about 6000 A.

If the caesium covering the boundary zone between the portions A and Bwas lighted, m potential difference or electron current was perceived. IThe photoelectric substance constituting the electrode 9 may beadvantageouslyformed, not

' only from an alkaline metal such as caesium, but

also from an alkaline earth metal. Good results can be obtained, forexample, with barium applied to a layer 6 of barium oxide. The bariumoxide," which constitutesin this case the intermediate rium vapor isintroduced again into the envelope or formed therein whereby theresettles on the barium oxide a film of metallic barium which constitutesthe photoelectric electrode. The oxidation of the barium to form theintermediate or separating layer, may also be eflfected bysuperfi'cialfy oxidizing, before the precipitation of the barium, theelectrode on which this barium is deposited. When this electrode isheated after or during the deposition of the barium, the latter willcombine with the oxygen of the oxide formed at the surface of theelectrode.

The intermediate or separating layer ii may also be formed in other waysthan in the manner above described and it need not always consist of acompound of the metal constituting one of the electrodes. Thus, forexample, the intermediate layer of insulating material may consist ofcalcium fluoride, which may be applied by evaporation to a metal plateconstituting one of the electrodes. If, subsequently, this calciumfluoride layer is brought into contact with caesium, the

latter will adsorb as a thin film to the calcium fluoride. By removingthe excessive caesium from saidsubstance may be causedto adsorb to theseparating layer by first coating the substance essentially constitutingthe intermediate or separating layer with a thin layer of a substancewhich adsorbs the photoelectric substance quite well. well adsorbingsubstance with one or more other substances which constitute theintermediate layer. In the above-described device in which theseparating layer consists of zirconium oxide said layer may be coated,for example, with a thin layer of caesium oxide. This oxide may beformed, for example, by depositing caesium on the zirconium oxide and bysubsequently admitting oxygen. After removal of the excessive oxygen anew quantity of caesium may be introduced into the envelope wherebythere is adsorbed to the caesium oxide layer a caesium film whichconstitutes the photoelectric electrode. The sensitivity of such adevice, 1. e. the potential diflerence produced with a given lighting,is very large.

Sometiines it is desired to decrease the electric resistance of theseparating layer. This-may advantageously be done by providing in theintermediate layer particles of an electrically conducting substance. Inthe case above set forth, in which the intermediate layer consists ofcalcium fluoride, it is possible, for example, to mix metal particleswith the calcium fluoride. If this sub stance is deposited byevaporation, a metal, for example, tungsten, may be evaporatedsimultaneously with the calcium fluoride in .which case- Sometimes it isalso possible to. mix said aoraeaa ment is heated, whereupon the calciumfluoride reacts with the tungsten and forms metallic calcium andtungsten fluoride both of which evaporate and settle on the electrode 3.The calcium and the tungsten fluoride reactagain with one another so asto form calcium fluoride and tungsten, the latter being present in thiscase in the calcium fluoride in a finely divided state.

Conveniently, the conducting particles may be particles of thephoto-active metal itself. For example, after caesium has been depositedon the intermediate layer consisting of calcium fluoride, the envelopeand the electrode system may be heated, whereby part of the caesiumpenetrates into the intermediate layer and the electrical resistance ofthis layer decreases whilst, in addition, the value of the potentialdifierence produced by the irradiation is favourably influenced.

' What we claim is:

1..A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal whichhas a work function no greater than two and one half volts and selectedfrom the group including the alkali metals and cadmium, barium andcalcium, a thin separating layer of solid insulating particlesinterposed between and in contact with substantially the entire areas ofsaid electrodes, and electrical conductors respectively contacting andconnected to said electrodes.

2. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said Wall and comprising a film of alkali metal, athin separating layer of solid insulating particles interposed betweenand in contact with substantially the entire areas of said electrodesand intimately adjoined to said film, and electrical conductorsrespectively con-' tacting and connected to said electrodes.

3. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work function no greater than two and one half volts, a thinseparating layer of particles of an insulating chemical compound of themetal of one of said electrodes interposed between and in contact withsubstantially the entire areas of said electrodes, said compound radicalbeing chosen from a group including fluorine and oxygen, and electricalconductors respectively contacting and connected to said electrodes.

4. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work function no greater than two and one half volts, a-thinseparating layer of solid insulating particles interposed between and incontact with substantially the entire areas of said electrodes and towhich said film is at least partly adsorbed, and electrical conductorsrespectively contacting and connected to said electrodes.

5. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall, said photoelectric electrode beingselected from the group including the alkali and the alkaline earthmetals and comprising a monomolecular film of a metal which has a workfunction no greater than two and one half volts, a thin separating layerof solid insu lating particles separating and in contact withsubstantially the entire areas of said electrodes,

and electrical conductors respectively contacting and connected to saidelectrodes.

6. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work ,function no greater than two and one half volts, 2. thinseparating layer interposed between and in contact with substantiallythe entire areas of said electrodes and composed of solid insulatingparticles interspersed with conducting particles, and electricalconductors respectively contacting and connected to said electrodes.

7. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work function no greater than two and one half volts, a thinseparating layer interposed between and in contact with substantiallythe entire areas of said electrodes and composed of solid insulatingparticles with particles of the metal of said photoelectric electrodeinterspersed in said layer, and electrical conductors respectivelycontacting and connected to said electrodes.

8. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work function no greater than two and one half volts, a thinseparating layer of solid insulating particles interposed between and incontact with substantially the entire areas of said electrodes, a filmof alkali metal oxide interposed between and adsorbed to said layer andto said photoelectric electrode, and electrical conductors respectivelycontacting and connected to 1 said electrodes.

9. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of a metal selectedfrom the group including the alkali and the alkaline earth metals whichhas a work function no greater than two and one half volts, a thinseparating layer of solid particles of an insulating binary compound ofan alkaline earth metal interposed between and in contact withsubstantially the entire areas of said electrodes, and electricalconductors respectively contacting and connected to said electrodes.

10. A photoelectric device comprising a sealed vessel with a transparentwall and enclosing a metal plate electrode, a cooperating photoelectricelectrode exposed to said wall and comprising a film of caesium, a thinseparating layer of particles of calcium fluoride interposed between andin contact with substantially the entire areas of said electrodes, andelectrical conductors respectively contacting and connected to saidelectrodes. 11. A photoelectric device comprising a sealed vessel with atransparent wall and enclosing a I zirconium plate electrode, acooperating photoelectric electrode exposed to said walland comprising afilm of a metal selected from the group i including the alkali and thealkaline earth metals which has a work function no greater than two andone half volts, a thin separating layer of zirconium oxide on thesurface of said plate electrode and located between and in contact with20 said wall and comprising a film 01' alkali metal adjoining andsupported by said layer, and electrical connections to said electrodes.

/ 13. A photoelectric device comprising a sealed zirconium electrodehaving on its surface a layer a fraction of amillimeter thick 01'zirconium oxide, a transparent film or caesium on said layer, andelectrical connections to said electrodes.

14. A-photoelectric device comprising a sealed vessel with a transparentwall, a metal plate electrode therein, one entire face of which isoxidized,

a layer of caesium in contact with all of the on dized portion of-themetal plate electrode and exposed to said transparent wall, and metallicconductors respectively contacting and connected to the metal plateelectrode and the layer of caesium.

JAN HENDRIK n: BOER. WILLEM CHRISTIAAN VAN GEEL.

vessel with a transparent wall and enclosing a l

